Civil Engineering Reference
In-Depth Information
Overconsolidated clays
With this soil its weakest strength occurs once the soil has reached its drained state. The operative param-
eters are therefore c
′
and
φ
′
, although this is an over-simplification for the case when the level of soil in
front of the wall has been reduced by excavation. In this instance there will be a relief of overburden
pressure which could result in softening occurring within the soil. When this happens some estimation of
the strength reduction of the soil must be made, possibly by shear tests on samples of the softened soil.
Silts
As for active pressure, the passive resistance of a silt can be estimated either from the results of
in situ
penetration tests or from a drained triaxial test. For passive pressure the appropriate strength parameters
are c
′
and
φ
′
.
Exercises
Exercise 7.1
A 6 m high retaining wall with a smooth vertical back retains a mass of dry cohesion-
less soil that has a horizontal surface level with the top of the wall and carries a
uniformly distributed load of 10 kPa. The soil weighs 20 kN/m
3
and has an angle of
shearing resistance of 36°.
Determine the active thrust on the back of the wall per metre length of wall (i)
without the uniform surcharge and (ii) with the surcharge.
Answers
(i) 93.6 kN; (ii) 109.2 kN
Exercise 7.2
The back of a 10.7 m high wall slopes away from the soil it retains at an angle of
10° to the vertical. The surface of the soil slopes up from the top of the wall at an
angle of 20°. The soil is cohesionless with a density of 17.6 kN/m
3
and
φ
′
=
33°.
If the angle of wall friction,
δ
=
19°, determine the maximum thrust on the wall:
(a) graphically and (b) analytically using the Coulomb theory.
Answer
(a) and (b) P
a
=
476.3 kN
Exercise 7.3
The soil profile acting against the back of a retaining wall is shown in Fig.
7.32.
Assuming that Rankine's conditions apply, determine both the theoretical lateral
earth pressure distribution and the distribution that would be used in design, and
sketch the two pressure distribution diagrams.
From the pressure distribution that would be used in design, determine the
magnitude of the total thrust that acts on the wall.
Determine the point of application of the total thrust, and express it as the dis-
tance from the base of the wall.
Answer
P
=
183.6 kN; x
=
2.56 m above base.
